Technical Field
The present invention relates to a transmission technique and a communication system including implementation of a Differential Space-Time Block Coding (DSTBC) method relating to transmission diversity in radio communication.
Related Art
One means for improving the reception quality of a mobile station during communication with the mobile unit includes a transmission diversity method using Space-Time Block Coding (STBC). The STBC can implement transmission diversity in a transmitter without using CSI (Channel State Information). Therefore, the STBC is effective under a fading environment, but requires the CSI at the receiver side.
The CSI is an element given to a signal when the radio signal propagates through space, and when a change in the CSI is small, the CSI can be estimated at a reception side by using a training signal. However, in a communication system having a narrow transmission frequency bandwidth and moving at a high speed, the parameter of the CSI changes while training is performed. Therefore, this issue is a significant problem.
Therefore, the DSTBC method that does not require the CSI in a transmitter and a receiver has been suggested by Tarokh et al. In the DSTBC method, the CSI is not required at the receiver either. Therefore, the DSTBC method is effective even under a fast fading environment that cannot be tracked with the STBC.
For example, a train radio system including multiple base stations provided along a railway track and simultaneously transmitting signals using a same frequency so that a single radio area is structured, and multiple vehicle stations receiving the signals in a broadcast manner. In this train radio system, at around the middle point of two adjacent base stations, two identical signals arriving from opposite directions interfere with each other at about the same power so that a standing wave is formed which may greatly degrade the communication quality. This phenomenon is called identical wave interference.
Therefore, an application of a DSTBC method has been suggested (JP 2012-015682 A) as a countermeasure against the identical wave interference in train radio systems. This employs the DSTBC method for the downlink of radio communication between a base station and a mobile station (i.e., the base station to the mobile station), and two orthogonal code sequences (hereinafter referred to as a sequence A and a sequence B) are generated and used. This is a method, in which a base station transmitting only the sequence A and a base station transmitting only the sequence B are arranged alternately, so that even at around the middle point between two base stations, the incoming waves from the two base stations do not interfere with each other, and on the contrary, transmission diversity effect can be obtained, and the communication quality can be improved. This will be explained with reference to FIG. 7.
FIG. 7 is an example case of train radio systems, and the following explanation is focused on the downlink. In order to simplify the explanation, a system constituted by two base stations is used in this model.
A train radio system of FIG. 7 includes a dispatch console 64, a central apparatus 65, a base station 61, a base station 62, and a vehicle station 63. The dispatch console 64 performs control of the central apparatus 65. For example, a wire circuit 66 such as an optical fiber connects between the central apparatus 65 and base stations 61 and 62. A radio circuit of the same frequency connects between the base station 61 and the vehicle station 63 and between the base station 62 and the vehicle station 63. Therefore, a broadcast call with audio can be made between the dispatch console 64 and the vehicle station 63.
It should be noted that the radio area in the model case of FIG. 7 is structured as follows.
More specifically, the transmission radio wave from the base station 61 covers the range of a coverage area CA1 of FIG. 7, and the transmission radio wave from the base station 62 covers the range of a coverage area CA2 of FIG. 7. The coverage areas CA1 and CA2 overlap each other at around the border between the coverage areas CA1 and CA2 in order to eliminate blind zone. As a result, the following areas exist: an area 81 in which the radio wave of the base station 61 can be received at a certain level or higher, an area 82 in which the radio wave of the base station 62 can be received at a certain level or higher, and an area 83 in which the radio waves of both of the base stations 61 and 62 can be received at a certain level or higher.
The train radio system of FIG. 7 is a system to which the DSTBC coding method is applied in order to cope with identical wave interference that occurs in the area 83. More specifically, the base station 61 having a DSTBC code transmitter 61a provided therein transmits only the sequence A of DSTBC, which covers the coverage area CA1. Likewise, the base station 62 having a DSTBC code transmitter 62a provided therein transmits only the sequence B of DSTBC, which covers the coverage area CA2. The vehicle station 63 has a DSTBC decoding receiver 63a provided therein, and communicates by receiving and decoding radio waves arriving from the base station 61 and the base station 62.
In this case, a generally available DSTBC decoding receiver has the following features.
The feature 1 is that even if the receiver receives only the sequence A (area 81 in FIG. 7), or only the sequence B (area 82 in FIG. 7), or the composite wave of the sequence A and the sequence B (area 83 in FIG. 7), the receiver is able to decode all of them with the same circuit configuration.
The feature 2 is that when the receiver receives the composite wave of the sequence A and the sequence B, e.g., in the area 83, the receiver can obtain the effect of transmission diversity regardless of the phase relationship of both of them.
Because of the above features, the example case of FIG. 7 can be said to be an effective method as a countermeasure against the identical wave interference.
However, as a method for coping with the identical wave interference, the system of FIG. 7 employs a method of transmitting only one of the sequences of DSTBC from a single base station. In this method, there would no problem in the interference area (area 83) of the two base station radio waves, but there is a problem in that, in an area where the receiver receives only one of the two sequences or in an area where one of the electric field strengths is significantly low (the areas 81 and 82), the communication quantity is slightly degraded as compared with the case where the DSTBC is not applied.